It is well known in the adhesives art that significant improvement in adhesion to various substrates can be obtained by incorporating various adhesion promoters into adhesive compositions for coating the substrate. Development of a wide-ranging adhesives art has benefited from this method of adhesion improvement and various organosilanes have been typically employed as the adhesion promoter.
Organosilanes which are useful in these applications generally contain hydrolyzable groups (e.g., halogen, alkoxy) attached to the silicon atom thereof which generate silanol groups upon contact with ambient moisture, and thus readily form chemical and/or physical bonds with mineral and metal surfaces. Also attached to the silicon of the organosilane adhesion promoter is an organic moiety which is reactive with, or at least shows some affinity towards, one of the components of the adhesive (usually the polymer). This latter requirement usually limits the practical utility of a specific organosilane to particular adhesive-substrate combinations. In this way, a chemical or physical "molecular bridge" is believed to be formed between the adhesive and the substrate which results in the observed macroscopic improvement in adhesion.
Thus, for example, U.S. Pat. No. 3,644,245 to Flanagan et al. discloses hot melt adhesive compositions having improved adhesion under conditions of high humidity. These compositions comprise a synthetic polymer base and hydrolyzable silanes containing such organic groups as phenyl, vinyl, epoxy, mercapto, amino, ethyl, methyl and methacryloxypropyl.
Additionally, the adhesion to a substrate, as well as the cohesive strength of an adhesive composition, may be augmented by at least partially cross-linking the adhesive. Various examples of this method exist in the art, which method is illustrated by U.S. Pat. No. 3,657,379 to Hilbelink et al. This disclosure teaches adhesive systems comprising separated reactive components wherein (1) a first reactive polymer is mixed with a curing agent for a second reactive polymer to yield one component and (2) a second reactive polymer is mixed with a curing agent for the first reactive polymer to form another component. When the reactive materials from the two components are mixed, the curing agents react with their respective reactive polymers to give strong adhesive bonds.
A distinct category of adhesives which generally comprises natural or synthetic rubbers and resin tackifiers is known in the art as "pressure-sensitive adhesives" (PSAs). PSAs uniquely maintain a permanent "tack" which provides a bond of measurable strength immediately upon contact with another surface. Again, adhesion to a substrate, as well as the cohesive strength of the PSA composition itself, can be improved by curing the PSA.
A cured PSA based on a polyether having at least one silicon-containing hydrolyzable group in its molecule is disclosed by Hirose et al. in U.S. Pat. No. 4,463,115. This composition also contains a tackifier and, optionally a silanol condensation catalyst for accelerating the curing reaction. It is stated that the resulting PSA has good adhesive properties and can be prepared substantially without the use of a solvent.
In a subsequent U.S. Pat. No. 4,665,127, Hirose et al. extended the above concept to include a wide variety of polymers having at least one reactive silicon-containing group combined with an organic aluminum or organic zirconium catalyst. These PSA compositions are said to have excellent heat resistance and releasability from substrates such as a silicone-coated release paper.
In U.S. Pat. No. 3,707,518, Bemmels et al. disclose a self cross-linking pressure-sensitive acrylate adhesive and tape which comprises a copolymer that includes a small amount of an alkoxy silyl polymerizable crosslinking monomer. Also included in these compositions is 2-6 parts by weight of an acid monomer which acts as a built-in catalyst and causes the adhesive copolymer to cure at ordinary temperatures.
Aside from silicone-based PSAs and the somewhat related above mentioned examples wherein silanes are used as adhesion promoters or compositions wherein reactive silicon groups are chemically attached to organic molecules to promote cure therebetween, silicones are generally known in the art to be abhesive materials (i.e., they impart good release of adhesives). Indeed, they have been widely employed in release liners for pressure-sensitive tape. Thus, for example, in U.S. Pat. No. 4,322,518, assigned to the assignee of the present invention. Blizzard discloses silicone coating compositions which comprise a curable silicone polymer fluid or gum and a liquid SiO.sub.2 -based resin as the sole curing agent for the fluid or gum. When cured on various substrates, these silicone compositions provide release coatings having controllable release forces for adhesives applied thereto.